Microplates are commonly used for holding products, often liquids, for laboratory testing. A microplate is generally formed as a substantially flat plate, with multiple “wells” formed in the plate, which are used as relatively small-volume test-tubes in laboratory experiments and processes. Such microplates are used in analytical research and clinical diagnostic testing laboratories and can commonly be used in the enzyme-linked immunosorbent assay, which is a widely used method of modern medical diagnosing testing. Sample tubes are often used in similar circumstances and can be held in an array in a rack or holder arrangement to provide an array of sample vessels arranged similarly to wells of a microplate.
Wells in a microplates typically hold between a few tens of nano-liters, up to several milliliters of a liquid. Microplate wells can be of many shapes, having typically circular, oval, square, or rectanguloid openings as viewed from a direction perpendicular to the plate.
The types of products, fluids, emulsions or liquids stored and tested in the wells can vary in viscosity, composition, volume and in the number of separate components or ingredients making up the liquid. In some applications, the liquid added to the wells is pre-mixed and is simply provided to the wells in a ready-mixed state. Mixing during the time in which the liquid is in the wells may still be desirable to prevent separation of components in the liquid, for example. In other applications, a number of different liquids, products or ingredients may be added to wells of the plate in separate steps and in this case, it is often desirable to mix the products after they have been added to the micro plate. In other applications, it is desirable to mix the liquids either after they have been added to the micro plate in order to ensure that the contents do not separate over time, or to ensure that any reaction or treatment of the ingredients in the plate happens in a uniform manner throughout the volume of each well.
A number of solutions to the problem of in-well mixing have been proposed. These include some of the following technologies. Magnetic fields can be used to rotate rotatable metallic members, which are suspended in a rotatable configuration in each well. Metallic balls may be levitated and allowed to fall in the liquid to agitate it, by the use of magnetic fields. In this case, only the magnetic ball is in contact with the fluid. Pin members may be used to transfer ultrasonic vibrations into the liquids to mix them. Pulsating gas streams may also be use to agitate the liquids in the wells. Other contact-free, methods exist, in which the plate itself may be agitated by orbital movement of the plate to induce agitation due to inertia in the liquids in the wells. In other applications, surface acoustic waves (SAW) can be used to induce circulating currents in the well plates. Adaptive focused acoustics (AFA) are an alternative to surface acoustic waves. AFA act to focus acoustic vibrations onto a sample vessel from outside the sample vessel by use, for example, of a concave acoustic transducer, where the focus point of the acoustic waves generated is located at a focal point, the sample vessel being located at the focal point of the waves. The above arrangements have certain drawbacks and are not suited to all circumstances in which well-plate mixing is necessary. Similar issues apply in the mixing of samples in other vessels, such as sample tubes, which are often carried in arrays in a similar form or layout to wells of laboratory well plates. The present invention therefore seeks to overcome certain drawbacks of the prior art.